Apoptosis or programmed cell death is a regulated network of biochemical events which lead to cell death. It is a physiological process involved in cell differentiation, organ development and maintenance of cellular populations in multicellular organisms (Cohen, J. J., Immunol. Today 14: 126-130, 1993). Furthermore, apoptosis is a reaction to various external stimuli and cell damage (e.g. induced by drugs).
Apoptotic cells generally shrink and are phagocytosed by other cells. In contrast, necrotic cells are characterized by swelling, especially of the mitochondria which become dysfunctional, which usually results in cell lysis.
Molecular events characteristic of apoptotic cells include nuclear collapse with condensation of chromatin and loss of nucleoli. Later, the chromatin becomes fragmented into units of single or multiple nucleosomes which present a "ladder" appearance when separated by size on a gel matrix (Compton, M. M., Cancer Metast. Rev. 11: 105-119, 1992). Activation of an endogenous endonuclease causes the chromatin fragmentation. Intracellular RNA, especially mRNA, is also degraded early during apoptosis.
Apoptosis can be triggered in various ways, including virus infection, growth factor withdrawal, DNA damage resulting from irradiation, exposure to glucocorticoids and certain chemotherapy drugs, or by signals such as TNF binding to its receptor or crosslinking the Fas receptor with anti-Fas antibodies (Cohen, J. J., Immunol. Today 14:126-130, 1993; Williams, G. T., & Smith, C. A., Cell 74:777-779, 1993; Suda et al., Cell 75:1169-1178, 1993; Smith, et al., Cell 76:959-962, 1994; Lowe et al., Nature 362:847-849, 1993; Sentman et al., Cell 67:879-888, 1991). The mechanism of apoptosis is not well understood, but the observed molecular changes that occur in apoptotic cells suggest that endogenous genes are responsible for apoptosis. The proteins produced from these induced genes lead to destruction of RNA and DNA ultimately leading to cell death.
Just as cell death by apoptosis is involved in normal regulation of cellular populations, cell proliferation is also required to maintain homeostasis of tissues and organs. However, in some cells, proliferation is aberrant leading to cancer. Cancer cells can be invasive, metastatic and highly anaplastic.
Although the mechanisms of tumor formation are still not completely and well defined, genetic elements, including oncogenes, have been shown to increase cell proliferation and relieve cells of normal check-points in the cell division cycle. Genes that control cell cycle check points have been identified in multicellular organisms and in yeast where they play an essential role in regulating the cell cycle. It is even possible that cell cycle check points may serve as switch points for choosing between cell proliferation and apoptosis. Thus, when a gene that controls a checkpoint is deleted, mutated, amplified in the genome, or otherwise aberrantly expressed in the cell, it may divert the cell into aberrant proliferation. Similarly, when a gene that normally controls a cell cycle switch point leading to apoptosis is aberrantly expressed, it may result in abnornal cell proliferation.
Some mammalian genes and proteins that have been simultaneously implicated in the regulation of cell proliferation and apoptosis including the genes for p53, BCL-2 or Myc. Although the pathways leading to apoptosis have not been fuilly elucidated, several genes that play a role in apoptosis have also been shown to play an important role in cancer. The p53 gene, coding for a tumor suppressor, is required for radiation-induced apoptosis (Lowe et al., Nature 362:847-849, 1993). BCL-2 inhibits apoptosis in many cells (Sentman et al., Cell 67:879-888, 1991; Vanhaesebroeck et al., Oncogene 8: 1075-1081, 1993) and furthermore, increased BCL-2 gene expression has been detected in primary breast cancer tissue without bcl-2 gene amplification (Nathan et al., Ann. Oncol. 5:409-414, 1994).
Apoptosis can also be induced by exposing cells to Diphtheria toxin (DT) or Pseudomonas toxin (PE) (Kochi, S. K., and Collier, R. J., Exp. Cell Res. 208: 296-302, 1993; Chang, M. P., et al., J Biol. Chem. 264: 15261-15267, 1989; Morimoto, H., and Bonavida, B., J. Immunol. 149: 2089-2094, 1992). Both of these bacterial toxins inhibit eukaryotic protein synthesis by inactivating elongation factor 2 by ADP-ribosylation (Carrol, S. F. and Collier, R. J., J Biol. Chem. 262: 8707-8711, 1987). Although the toxin-specific mechanism by which these toxins induce apoptosis is unknown, it is not simply due to inhibition of protein synthesis because other protein synthesis inhibitors do not induce apoptosis (Chang, M. P., et al., J. Biol. Chem. 264: 15261-15267, 1989; Morimoto, H., and Bonavida, B., J. Immunol. 149: 2089-2094, 1992).